Technical Field
The present description relates to techniques for coupling light to integrated devices.
Description of the Related Art
For Wavelength Division Multiplexing (WDM), especially CWDM (Coarse WDM), applications one of the major concerns related to Silicon Photonics, or VHIC (Very High Index Contrast) technologies in general, is the bandwidth limitations of the grating coupler (GC) used to couple light in and out of the integrated devices. Depending on the nature of light the grating couplers are intended for, two kinds of grating coupler can generally be considered: Single Polarization Grating Couplers (SPGC) and Polarization Splitting Grating Couplers (PSGC). A 1 dB bandwidth for SPGC and PSGC is typically in the range of 20-25 nm. CWDM applications target 4λ with separation of 20 nm, so the total wavelength span is 60 nm requiring a total input/output bandwidth of approximately 80 nm to take into account possible source wavelength variations, technological tolerances and temperature dependences.
The main limitation to the bandwidth is not related to the GC itself, but in the system collecting/injecting light from/to the grating coupler. Considering the Grating Equation shown herebelow, it is clear that different wavelengths are emitted/accepted at different angles
                              1          Λ                =                              1            λ                    ⁢                      (                                          n                eff                            -                              sin                ⁡                                  (                  θ                  )                                                      )                                              (        1        )            where Λ is the grating period, neff the effective waveguide index and θ the emission/insertion angle with respect to the normal of the grating plane (XY in FIG. 1).
Typically a Single Mode Fiber (SMF) can collect/inject optimally only one wavelength, due to the fact that it is positioned at a fixed angle and position with respect to the CG.
In FIG. 1 it is schematically shown a system 10 with conventional coupling, where, given an horizontal plane defined by orthogonal horizontal axes X and Y, a grating coupler 11 lies in such XY plane and receives incident light 12 along axis Y, for instance from a laser source, and emits by diffraction a emitted light 12R at different emission angles with a given angle distribution according to the different wavelengths λ1, λ2, λ3, as defined by the Grating Equation (1) above, in the general direction of an integrated optic device represented by a fiber 13, for example a SMF fiber, which is, in FIG. 1, arranged as angled fiber, with its main longitudinal axis not parallel to the axis Y of the grating coupler 11. As shown in FIG. 1 only light at wavelength λ2 is optimally collected.
Thus, in conventional GC to SMF coupling, due to wavelength dependent emission angle, the collecting system is bandwidth limited.